All car speedometers have a 10% tolerance due to the equipment being mass produced. Therefore if you're doing 70 mph 10% is 7 mph, plus they then add an additional 2 mph to remove any doubt and ensure the 10% tolerance can't be used as an excuse. 70+7+2=79. 79 mph is the minimum speed they will prosecute you for being over. The 73 refers to the tolerance of the equipment, so they can't stop you and accuse you or doing 72 in a 70 when the device has an inbuilt tolerance of 3 mph.

it all depends if you drive bad or safe at 80mph, I have passed un- marked cars at 80 and just been pointed at, when the conditions were fine but seen people getting pulled when doing same when conditions were bad. its up to the assessment of the situation by the person who is there to monitor safety I have been in the middle lane doing over the limit and a guy in the outside lane was cruising at 80 he got stopped by un-marked behind me

Sorry I've been working on the essay since noon so I'm probably off the ball - If somebody does 77mph, if a fixed speed camera clocks them at that speed then they don't get fined/points due to manufacturer tolerance? (10% tolerance seems massive btw)

Or is it just plain over 70 and you get points on your license - which can't be right if speedometers have a 10 percent tolerance because then somebody could unknowingly be doing 77?

The police don't care for care for the tolerances built into speedos or GPS units or anything.
But as I said your speedo is not allowed to read under. So if you are 1mph over the limit and PC plod is having a bad day you can get points. But it is very unlikely. Section 9.2 in the PDF I linked explains this.
Speeding is an absolute offence, there is no mitigation. But the guidelines are there to provide a little bit of clarity.

Okay makes a lot more sense now, so basically:
70-73 - pulled over and fined by a policeman BUT can be challenged successfully by citing the PDF
70-73 - fixed speed cams won't bother?
73+ - fine + pts

Assuming I've finally got it, how can somebody be sure their speedo is accurate. For example my car was hopefully accurate when it left the showroom 15 yrs ago but now is it still accurate seeing it has never been tested/calibrated by MOT testers?

The guidelines are that you won't be fined until you reach 79, by either a policeman or speed camera.

But you can be if the police want to. A lot of it is down to how you are driving.
But you won't be allowed to use the guidelines as a defence.
Only way you can get an idea of your speedo accuracy is by using a sat nav.

Speedo are only allowed a 8% tolerance as per the vehicle approval regulations. But more importantly they are not allowed to read under, thus the reason for the tolerance.

Click to expand...

I heard 10% several years ago so it would make sense that the tolerances have been tightened up since then. But 10% is what's still used in the table. I assume this is because there's still a lot of older cars on the road with less accurate speedo's.

The Accord speedo seems to be quite accurate. I've compared mine to the factory sat nav and it's only reading 2 mph over at 60mph.

Building on what Rob brought up, even if you did get fined for doing just a few miles over the limit - or within the given tolerances for the equipment they used, it wouldn't be hard to contest the fine.

Another note, if it's an actual bobby issuing a ticket, you can also not accept it. Signing the ticket is agreeing to a contract stating that you are willing to pay a fine, like all police documents it's written in legalese (where must is synonymous with may, order with offer etc. - that's no joke).

Sometimes they'll just leave you to it, sometimes they'll ask you to come down to the station, but it's worth fighting for.

It's at the digression of the officer from that I understand. If they haven't got the proper evidence or there know there's room to contest it, they're are likely to not push as much. But again as you say, unless you've pissed them off.

You can also write to the local highway authority and request a copy of the traffic order for the exact location, which will tell you whether the limit there has actually legally been set (you'd be surprised how many signs go up and the right paperwork isn't done).